absorção de amonia em um sistema refrigerado

8/10/2019 Absoro de Amonia Em Um Sistema Refrigerado

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Procedia Computer Science 19 ( 2013 ) 754 761

1877-0509 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of Elhadi M. Shakshukidoi: 10.1016/j.procs.2013.06.099

The 3 rd International Conference on Sustainable Energy Information Technology(SEIT 2013)

Integration of an ammonia-water absorption refrigerationsystem with a marine Diesel engine: A thermodynamic study

Ahmed Ouadha*, Youcef El-GotniLaboratoire dEnergie et Propulsion Navale, Facult de Gnie Mcanique,

Universit des Sciences et de la Technologie Mohamed BOUDIAF dOran,B.P. 1505 Oran El-Mnaouar, 31000 Oran, Algrie

Abstract

This paper examines through a thermodynamic analysis the feasibility of using waste heat from marine Dieselengines to drive an ammonia-water absorption refrigeration system. An energy balance of a diesel engine shows thatsufficient waste heat is provided. The results illustrate that higher performance of the system is obtained at highgenerator and evaporator temperatures and also at low condenser and absorber temperatures.

2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer]

Keywords: Ammonia-water; Absorption refrigeration; Marine Diesel engine; Thermodynamic

1. Introduction

As fuel prices soar, marine diesel engines manufacturers are beginning to scout for more energy-efficiency solutions. This can be achieved by increasing the engine efficiency and developingtechnologies with low pollutant emissions. The increase of the diesel engine efficiency can be achieved byvalorizing the waste thermal energy. Onboard ships, a considerable amount of primary energy may besaved by valuing the waste heat rejected during the operation of the main propulsion systems.

* Corresponding author. Tel.: +213 661 204 325; fax: +213 41 290 466. E-mail address : [email protected]

Available online at www.sciencedirect.com

2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of Elhadi M. Shakshuki

Open access under CC BY -NC-ND license.

Open access under CC BY -NC-ND license.
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756 Ahmed Ouadha and Youcef El-Gotni / Procedia Computer Science 19 ( 2013 ) 754 761

2. Marine Diesel engine energy balance

Although the great technological development of modern marine Diesel engines, only a part of theenergy contained in the fuel is converted to power output. The maximum efficiency remains lower than45%. The main losses are dissipated as heat in the exhaust gases, coolants, and transferred to theenvironment.

An energy balance of a marine Diesel engine indicates how the energy contained in the fuel is used orlost. Indeed, the injection of a mass of fuel in the hot air in the combustion chamber produces a largeamount of heat. However, the mechanical work requires only a fraction of the energy produced. Theresidual energy is, in fact, discharged at various places during his stay in the cylinder. For a marine Dieselengine, a first-law analysis yields

in c ex r Q W Q Q Q (1)

where, inQ is the heat supplied to the Diesel engine, W the mechanical energy output, cQ the heat

transferred to the cooling systems, exQ the heat rejected in exhaust gases, r Q the heat transferred to theenvironment by radiation.

Figure 1 shows an example of an energy balance of a modern marine Diesel engine. The heattransferred to coolants includes charge air cooling (17.8%), jacket water cooling (4.8%) and lubricatingoil cooling (3.2%). In addition, 25.1% of the total energy is lost, released into the atmosphere during theexhaust outlet. Finally, a small part of the energy (0.6%) is released by radiation. At first glance, theanalysis of heat flows shows that there are three engine waste heat streams, at different temperaturelevels, that have potential to be recovered: exhaust gas (300-600C); charge air (200C); jacket water (80-100C).

Fig. 1. Typical heat balance of a marine Diesel engine

3. Thermodynamic analysis

In order to carry out a thermodynamic analysis, the conservation laws of mass and energy have beenapplied to each component of the system. Every component has been considered as a control volumeexchanging heat, work and inflow and outflow streams with its surrounding. To simplify the theoreticalmodel, the following assumptions have been considered:

- The analysis is carried out under steady state conditions and assuming thermodynamicequilibrium at all points of the cycle;

- Ammonia at the generator and evaporator outlets is assumed as saturated vapour;


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- Ammonia at the condenser outlet is saturated liquid;- Pressure losses in the pipes and all heat exchangers are negligible;- Heat exchange release to surroundings does not occur.The absorption system considered uses ammonia as the refrigerant and water as the absorber to create

a continuous cycle for the ammonia. Figure 2 shows a schematic of a basic ammonia-water absorptionrefrigeration system. It consists mainly of a generator, a condenser, an evaporator, an absorber, a solutionheat exchanger and a circulation pump. The generator uses waste heat from the marine Diesel engine toseparate ammonia vapour from the concentrated ammonia solution. In the condenser, high pressureammonia vapours are cooled and condensed to liquid state. Liquid ammonia leaves the condenser andflows to the evaporator through an expansion valve. The refrigerant then enters the evaporator, where itreceives heat from the cold source. Then, ammonia vapour enters the absorber, where a weak solution ofwater and low concentration ammonia absorbs the refrigerant and, at the same time, releases heat to theneighbourhood. The ammonia-water solution flows back to the generator through a circulation pump toundergo a new cycle.

Fig. 2. Schematic diagram of an absorption refrigeration suystem driven by a Diesel engine waste heat

Based on the above assumptions, the governing equations for mass and energy conservation are given by the following expressions:

0i om m (2)

0i i o om x m x (3)

o o i iQ W m h m h (4)

where xi and xo correspond to the inlet and outlet ammonia mass fractions in the solution.The application of the mass and energy balance equations to the generator yields

7 1 8m m m (5)

7 7 1 8 8m x m m x (6)

1 1 8 8 7 7 g Q m h m h m h (7)

From equations (5) and (6), mass flow rates of strong and weak solutions can be calculated:

7

8 1

7 8

1 xm m x

(8)

8

7 1

7 8

1 xm m

x x (9)

Equation (9) allows to define the circulation ratio of the system:

7

1

m f

m (10)


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The energy balance of the solution heat exchanger yields

9 6 81T T T (11)

8

7 6 8 9

6

mh h h h

m (12)

where, is the solution heat exchanger effectiveness.The energy required to pump the solution from the low pressure (absorber) to the high pressure(generator) is:

6 5 6 pw p p v (13)

where v6 is the specific volume of the strong solution.Then, enthalpy at point 6 can be calculated:

6 5 6 5 6h h p p v (14)

Finally, the mass and energy balance equations applied to the absorber, the condenser and the evaporatorgive the following expressions:

4 4 10 10 5 5aQ m h m h m h (15)

1 1 2cQ m h h (16)

1 4 3eQ m h h (17)

The performance of refrigeration systems are usually measured using the coefficient of performance(COP). This parameter is defined as the ratio of the useful effect produced to the energy input of thesystem:

e

g p

QCOP

Q W (18)

4. Results and discussions

The analysis involves the determination of effects of generator, absorber and evaporator temperatureson the performance of the system. The performance parameters considered are coefficient of performanceand the circulation ratio.

A computer program has been prepared in order to calculate the performance of the ammonia-waterabsorption refrigeration cycle accordingly to the model presented in the above section. The program usessubroutines allowing the calculation of thermodynamic properties of the binary solution as function oftemperature and concentration [5]. Fixed data used in the simulation are summarized in Table 1. Theoperating condenser and absorber temperatures are 20 to 40C and depend on the cooling waterconditions. The solution heat exchanger effectiveness is defined as the ratio of the temperature drop ofstrong solution to the temperature difference of the strong and weak solutions entering the heat exchanger.

Table 1. Fixed data use in the simulation

Generator temperature, C 60-120Condenser temperature, C 20-45Absorber temperature, C 20-45Evaporator temperature, C -10-10Solution heat exchanger effectiveness, % 70 - 100Solution mass flow rate, kg/min 3.5Ammonia mass flow rate, kg/min 1


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Figure 3 shows the combined influence of generator and condenser temperatures on the COP and thecirculation ratio of the cycle for absorber and evaporator temperatures fixed to 25 and -5C, respectively.The effectiveness of the solution exchanger was set to 80%. Generally, the coefficient of performanceincreases with increasing the generator temperature as shown if Fig. 3. a . In addition, the increase in thecondenser temperature results in deterioration in the COP. It is noted that for every condensationtemperature, there is a lower limit of the generator temperature below which the cycle can not beachieved. It is interesting to have this limit as low as possible if the waste heat source used is the engine

jacket cooling water. The later has a temperature ranging from 80 to 100C. The circulation ratioincreases considerably for generator temperatures approaching their lower limits (Fig. 3. b). Higher valuesof the circulation ratio require unacceptable dimensions of solution pumps. This explains the impossibilityof achieving a cycle at very low generator temperatures. Increasing the condenser temperature in turnincreases the circulation ratio.a

50 60 70 80 90 100 110 120 1300.0

0.2

0.4

0.6

0.8

Tabs

= 25CTevap = -5C

= 80%

C o e

f f i c i e n t o f p e r f o r m a n c e

Generator temperature (C)

T cond = 40C = 30C = 20C

b

50 60 70 80 90 100 110 120 1300

10203040

5060708090

100T

abs = 25C

Tevap = -5C = 80%

Tcond

= 40C = 30C = 20C

C i r c u l a t

i o n r a

t i o


Fig. 3. Combined effect of generator and condenser temperatures on the performance of the system: a. COP; b. Circulation ratio

Figure 4 shows the variation of the COP and the circulation ratio according to the generatortemperature for various temperatures of the absorber. The fact that temperatures prevailing in the absorberand the condenser are in the same level, the effect of the absorber temperature is similar to that of thecondensation temperature on the COP and the circulation ratio.a

50 60 70 80 90 100 110 120 1300.0

0.2

0.4

0.6

0.8

Tcond = 25CTvap = -5C = 80%

Tabs = 40C = 30C = 20C C

o e f f i c i e n t o f p e r f o r m a n c e


b

50 60 70 80 90 100 110 120 1300

20

40

60

80

100

Tcond = 25CTevap = -5C = 80%

Tabs

= 40C = 30C = 20C

C i r c u

l a t i o n r a

t i o


Fig. 4. Combined effect of generator and absorber temperatures on the performance of the system: a . COP; b. Circulation ratio

The evaporator temperature is directly related to the temperature of the product to be cooled. Themain advantage of using ammonia-water mixture is to reach negative cooling temperatures. Fig. 5 showsthe combined effect of generator and evaporator temperatures on the coefficient of performance and thecirculation ratio. These curves have been obtained for fixed condenser and absorber temperatures and


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temperatures and also at low condenser and absorber temperatures. Furthermore, the increase in thesolution heat exchanger effectiveness improves the coefficient of performance with no effect on thecirculation ratio.

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absorção de amonia em um sistema refrigerado

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